Cytochrome P450 monoxygenases and cytochrome c oxidases are used to access the biological mechanism and role of oxygen binding heme proteins in electron transport. P450 cam, the preferred model, has been purified and crystallized in quantity from a procaryote source. The primary and tertiary structures emerge from two orthorhombic and one tetragonal crystal form. The space groups have been determined and unique isomorphous replacements prepared. The second orthorhombic crystal space group is P212121 with one molecule per asymmetric center, four per unit cell, and reflections suitable for 2.5 A or better resolution. The heme prosthetic group geometry has been established from single crystal polarized optical and EPR spectra. Physical properties via ENDOR show exchangeable protons in the low spin ferric form and eliminate nitrogen as an axial ligand in either the low or high spin species. Analysis continues on two c551 cytochromes of known primary sequence from related procaryotic strains that differ significantly in NMR spectra at 360 mHz. We continue to provide in significant quantity crystalline P450 cam for structure elucidation. Active collaborations in the cryobiochemistry of labile intermediates and equilibria with the Pierre Douzou group in Paris and with Peter Debrunner in Physics play a leading role in physical probe measurements and interpretation.